U.S. patent application number 13/743440 was filed with the patent office on 2013-07-18 for photovoltaic device having an absorber multilayer and method of manufacturing the same.
This patent application is currently assigned to FIRST SOLAR, INC. The applicant listed for this patent is FIRST SOLAR, INC. Invention is credited to Changming Jin, Xilin Peng, Rick C. Powell, Aaron Roggelin, Gang Xiong.
Application Number | 20130180579 13/743440 |
Document ID | / |
Family ID | 47604268 |
Filed Date | 2013-07-18 |
United States Patent
Application |
20130180579 |
Kind Code |
A1 |
Jin; Changming ; et
al. |
July 18, 2013 |
PHOTOVOLTAIC DEVICE HAVING AN ABSORBER MULTILAYER AND METHOD OF
MANUFACTURING THE SAME
Abstract
A photovoltaic device having an absorber multilayer and methods
of manufacturing the same are described. The absorber multilayer,
which is formed adjacent to a window layer, may include a doped
first cadmium telluride layer which contains a first dopant and an
intrinsic second cadmium telluride layer. The absorber multilayer
may further include at least a third cadmium telluride layer formed
adjacent to a back contact. The at least third cadmium telluride
layer may include doped or intrinsic cadmium telluride.
Inventors: |
Jin; Changming; (Sylvania,
OH) ; Peng; Xilin; (Bloomington, MN) ; Powell;
Rick C.; (Ann Arbor, MI) ; Roggelin; Aaron;
(Millbury, OH) ; Xiong; Gang; (Santa Clara,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FIRST SOLAR, INC; |
Perrysburg |
OH |
US |
|
|
Assignee: |
FIRST SOLAR, INC
Perrysburg
OH
|
Family ID: |
47604268 |
Appl. No.: |
13/743440 |
Filed: |
January 17, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61587171 |
Jan 17, 2012 |
|
|
|
Current U.S.
Class: |
136/255 ;
438/87 |
Current CPC
Class: |
H01L 31/075 20130101;
Y02E 10/543 20130101; H01L 31/073 20130101; H01L 31/1828
20130101 |
Class at
Publication: |
136/255 ;
438/87 |
International
Class: |
H01L 31/075 20060101
H01L031/075; H01L 31/18 20060101 H01L031/18 |
Claims
1. A photovoltaic device comprising: a window layer; a back contact
formed over the window layer; and an absorber multilayer formed
between the window layer and the back contact, the absorber
multilayer comprising: a doped first cadmium telluride layer which
contains a first dopant; and an intrinsic second cadmium telluride
layer.
2. The photovoltaic device of claim 1, wherein the doped first
cadmium telluride layer is formed between the window layer and the
intrinsic second cadmium telluride layer.
3. The photovoltaic device of claim 1, wherein the intrinsic second
cadmium telluride layer is formed between the window layer and the
doped first cadmium telluride layer.
4. The photovoltaic device of claim 1, wherein the first dopant
comprises a material selected from the group consisting of lithium,
sodium, potassium, rubidium, silicon, germanium, tin, copper,
silver, gold, nitrogen, phosphorus, arsenic, antimony, bismuth and
a chlorine-containing compound thereof.
5. The photovoltaic device of claim 4, wherein the first dopant
comprises rubidium or silicon.
6. (canceled)
7. The photovoltaic device of claim 1, the absorber multilayer
further comprising: at least one third cadmium telluride layer.
8. The photovoltaic device of claim 7, wherein the at least one
third cadmium telluride layer is formed between the back contact
and the doped first cadmium telluride layer.
9. The photovoltaic device of claim 7, wherein the at least one
third cadmium telluride layer is formed between the back contact
and the intrinsic second cadmium telluride layer.
10. The photovoltaic device of claim 7, wherein the at least one
third cadmium telluride layer contains a second dopant.
11. The photovoltaic device of claim 7, wherein the at least one
third cadmium telluride layer comprises intrinsic cadmium
telluride.
12. (canceled)
13. The photovoltaic device of claim 10, wherein the second dopant
comprises a material selected from the group consisting of copper,
silver, gold, nitrogen, phosphorus, arsenic, antimony, bismuth,
oxygen and a chlorine-containing compound thereof.
14. The photovoltaic device of claim 13, wherein the second dopant
comprises copper.
15. (canceled)
16. A method of forming a photovoltaic device, the method
comprising: forming a window layer over a substrate; forming an
absorber multilayer over the window layer, the absorber multilayer
comprising: a doped first cadmium telluride layer which contains a
first dopant; and an intrinsic second cadmium telluride layer.
17-22. (canceled)
23. The method of claim 16, wherein the step of forming an absorber
multilayer over the window layer further comprises forming at least
one third cadmium telluride layer.
24. The method of claim 23, wherein the at least one third cadmium
telluride layer is formed between the back contact and the doped
first cadmium telluride layer.
25-31. (canceled)
32. The method of claim 16, further comprising heating the absorber
multilayer at a temperature between about 380.degree. C. and about
800.degree. C. in the presence of cadmium chloride.
33-36. (canceled)
37. The method of claim 16, further comprising heating the
photovoltaic device to provide in-situ control of a thickness of
the window layer.
38. The method of claim 37, wherein the heating step comprises
heating the photovoltaic device at a temperature between about
450.degree. C. and about 800.degree. C.
39. The method of claim 38, wherein the window layer comprises
cadmium sulfide, and wherein the heating step further comprises:
reacting the first dopant with cadmium sulfide; and controlling the
window layer thickness to be greater than about 300 angstroms.
40. The method of claim 39, wherein the heating step further
comprises: forming intermediate compounds having melting points of
below about 450.degree. C.
41-43. (canceled)
Description
CLAIM OF PRIORITY
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Patent Application Ser. No.
61/587,171 filed on Jan. 17, 2012, which is hereby incorporated by
reference in its entirety.
TECHNICAL FIELD
[0002] Disclosed embodiments relate to the field of photovoltaic
devices, which include photovoltaic cells and photovoltaic modules
containing a plurality of cells, and method of manufacturing
thereof.
BACKGROUND
[0003] Photovoltaic devices such as photovoltaic modules or cells
may use a plurality of semiconductor materials as fundamental
layers in producing electric current. These fundamental layers may
include an n-type semiconductor window layer (e.g., cadmium
sulfide), and a p-type semiconductor absorber layer (e.g., cadmium
telluride). When photons pass through the n-type window layer and
are absorbed within the p-type absorber layer, electron-hole pairs
are generated. The interface of the n-type window layer and the
p-type absorber layer creates an electric field which separates
such electron-hole pairs to produce electric current.
[0004] Photo-conversion efficiency is the proportion of incident
photons that the photovoltaic device converts into electric
current. Various loss mechanisms can potentially diminish
photo-conversion efficiency. For instance, photons absorbed within
the window layer cannot be converted into electric current. In
addition, electrons can be lost through a process called
recombination, in which excited electrons in the conduction band
which would otherwise generate electric current are lost when such
electrons fall from the conduction band back into an empty state in
the valence band called a hole, or a position in the valence band
where an electron could exist.
[0005] Mitigating recombination improves the photo-conversion
efficiency of photovoltaic devices. A band gap is the difference in
energy between electron orbitals in the valence band and electron
orbitals in the conduction band. This difference is the amount of
electromagnetic energy required to excite an electron to the
conduction band to create a mobile charge carrier capable of
contributing to current flow in the photovoltaic device. Substances
with wide band gaps are generally insulators and those with
narrower band gaps are typically semiconductors. If an electron is
no longer in the conduction band, it will no longer contribute to
current flow. Thus, potential recombination interferes with current
flow in the device. Generally, a wider band gap adjacent to a back
contact, which can interface with the p-type absorber layer, can
help repel electrons away from the back contact to avoid
recombination.
[0006] To maximize the photo-conversion efficiency of photovoltaic
devices, it is desirable to minimize photon absorption within the
window layer and to mitigate recombination. A method of mitigating
such potential loss mechanisms and promoting photo-conversion
efficiency using an absorber layer is particularly desirable.
DESCRIPTION OF DRAWINGS
[0007] FIG. 1 is a cross-sectional view of a conventional
photovoltaic device.
[0008] FIG. 2A is a cross-sectional view of a photovoltaic device
according to a first embodiment at a stage of processing following
formation of a cadmium telluride multilayer.
[0009] FIG. 2B is a cross-sectional view of the photovoltaic device
of FIG. 2A at a stage of processing subsequent to that of FIG.
2A.
[0010] FIG. 3A is a cross-sectional view of a photovoltaic device
according to a second embodiment at a stage of processing following
formation of a cadmium telluride multilayer.
[0011] FIG. 3B is a cross-sectional view of the photovoltaic device
of FIG. 3A at a stage of processing subsequent to that of FIG.
3A.
[0012] FIG. 4A is a cross-sectional view of a photovoltaic device
according to a third embodiment at a stage of processing following
formation of a cadmium telluride multilayer.
[0013] FIG. 4B is a cross-sectional view of the photovoltaic device
of FIG. 4A at a stage of processing subsequent to that of FIG.
4A.
[0014] FIG. 5A is a cross-sectional view of a photovoltaic device
according to a fourth embodiment at a stage of processing following
formation of a cadmium telluride multilayer.
[0015] FIG. 5B is a cross-sectional view of the photovoltaic device
of FIG. 5A at a stage of processing subsequent to that of FIG.
5A.
[0016] FIG. 6A is a cross-sectional view of a photovoltaic device
according to a fifth embodiment at a stage of processing following
formation of a cadmium telluride multilayer.
[0017] FIG. 6B is a cross-sectional view of the photovoltaic device
of FIG. 6A at a stage of processing subsequent to that of FIG.
6A.
[0018] FIG. 7 is a schematic of a manufacturing process for a
photovoltaic device having a cadmium telluride multilayer.
DETAILED DESCRIPTION
[0019] In the following detailed description, reference is made to
the accompanying drawings which form a part hereof, and in which is
shown by way of illustration specific embodiments that may be
practiced. These embodiments are described in sufficient detail to
enable those skilled in the art to make and use them, and it is to
be understood that structural, logical, or procedural changes may
be made to the specific embodiments disclosed without departing
from the spirit and scope of the invention.
[0020] Embodiments described herein provide for a photovoltaic
device having a multilayered fundamental layer and methods of
manufacturing the same. The multilayered fundamental layer can
mitigate photon absorption and maximize the photo-conversion
efficiency within the photovoltaic device through recombination
mitigation. In the disclosed embodiments, the multilayered
fundamental layer used is the absorber layer. The multilayered
absorber layer (or absorber multilayer) includes at least a doped
first cadmium telluride layer and an intrinsic (i.e. substantially
free of dopant material at formation) second cadmium telluride
layer. Note that, although embodiments described herein include a
multilayered absorber layer having a doped cadmium telluride first
layer and an intrinsic cadmium telluride second layer, the
invention is not thus restricted. Any method that can be used to
mitigate photon absorption and maximize photo-conversion efficiency
is well within the realm of the invention. For example and as
described below, more than one doped absorber layer may be used in
conjunction with an intrinsic absorber layer. Hence, the use of a
multilayered absorber layer having a doped first cadmium telluride
layer and an intrinsic second cadmium telluride layer is only for
illustrative purposes.
[0021] Referring to FIG. 1, by way of example, a conventional
photovoltaic device 10 can be formed sequentially in a stack on a
substrate 100, for example, soda-lime glass or other suitable glass
or material. Because substrate 100 is not conductive, device 10 can
include a front contact 120, which can include a multi-layered
transparent conductive oxide (TCO) stack with several functional
layers including a barrier layer 112 to protect the semiconductor
layers from potential contaminants from substrate 100, a TCO layer
114 to provide for high optical transmission and low electrical
resistance, and a buffer layer 116 to mitigate potential
irregularities during the subsequent formation of the semiconductor
layers, for example. The barrier layer 112 may include, for
example, silicon dioxide. The TCO layer 114 may include any
suitable transparent conductive oxide, for example, cadmium
stannate or cadmium tin oxide. The buffer layer 116 may include
various suitable materials, for example, tin oxide (e.g., tin (IV)
oxide), zinc tin oxide, zinc oxide or zinc magnesium oxide.
[0022] The semiconductor layers can include an n-type semiconductor
window layer 130, such as a cadmium sulfide layer, formed on the
front contact 120 and a p-type semiconductor absorber layer 140,
such as a cadmium telluride layer, formed on the semiconductor
window layer 130. The window layer 130 can allow the penetration of
solar energy to the absorber layer 140, where the photon energy is
converted into electrical energy. Back contact 150 is formed over
absorber layer 140. Back contact 150 may be one or more highly
conductive materials, for example, molybdenum, aluminum, copper,
silver, gold, or any combination thereof, providing a
low-resistance ohmic contact. Front and back contacts 120, 150 may
serve as electrodes for transporting photocurrent away from device
10. Back support 160, which may be glass, is formed over back
contact 150 to protect device 10 from external hazards. Each layer
may in turn include more than one layer or film. Additionally, each
layer can cover all or a portion of the device and/or all or a
portion of the layer or substrate underlying the layer. For
example, a "layer" can include any amount of any material that
contacts all or a portion of a surface. It should be noted and
appreciated that any of the aforementioned layers may include
multiple layers, and that "on" or "onto" does not mean "directly
on," such that in some embodiments, one or more additional layers
may be positioned between the layers depicted.
[0023] Photons absorbed by the window layer 130 cannot be absorbed
by the absorber layer 140 which decreases the photo-conversion
efficiency of the device 10. One approach to mitigating light
absorption within the window layer 130, for example, is to decrease
its thickness at deposition. However, this has disadvantages. For
example, a window layer thickness that is less than 300 angstroms
(typical thicknesses range from 300 angstroms to 750 angstroms) is
so thin that the window layer 130 may have discontinuities in it.
For instance, the window layer 130 may provide only about 30% to
about 70% coverage of the front contact 120. Such limited coverage
of the front contact 120 results in intermittent and reduced
contact between the window layer 130 and the absorber layer 140
which can disrupt the local, built-in electric field within the p-n
junction formed at or near the interface of the p-type absorber and
n-type window layers 140, 130. When the p-n junction is disrupted,
non-uniform, unpredictable element diffusion across the p-n
junction can occur which increases the risk of diminished
electrical performance of the device 10. Current front contact 120
formation processes may generate a front contact 120 with a surface
roughness that can contribute to an increased risk of discontinuity
in the window layer 130 deposited thereon. Although the buffer
layer 116 may smooth out some of this roughness, it may be
insufficient when a thin window layer 130 is employed.
[0024] FIG. 2A illustrates a cross-sectional view of a first
embodiment of a photovoltaic device 20 at a stage of processing
after formation of a cadmium telluride absorber multilayer 270 in
lieu of absorber layer 140 (FIG. 1). Referring to FIG. 2A, rather
than depositing a window layer thinner than 300 angstroms, for
example, window layer 130 is formed and its thickness is controlled
in-situ to be greater than 300 angstroms, for example. Having the
thickness of the window layer be at least 300 angstroms greatly
minimizes the likelihood of discontinuities of the window layer
over the front contact 120.
[0025] Cadmium telluride multilayer 270 includes a doped first
cadmium telluride layer 142 and an intrinsic second cadmium
telluride layer 144. Cadmium telluride multilayer 270 may be formed
by vapor transport deposition, for example, as shown in FIG. 7 and
discussed below.
[0026] The doped first cadmium telluride layer 142 can include a
first dopant such as rubidium or silicon. More generally, the first
dopant can include a Group I-A dopant material, for example,
lithium, sodium, potassium, rubidium, cesium, a Group I-B dopant
material, for example, copper, silver, gold, a Group V-A dopant
material, for example, nitrogen, phosphorus, arsenic, antimony,
bismuth, a Group IV-A dopant material, for example, silicon,
germanium, tin and/or chlorine-containing compounds of the above
dopant materials. The aforementioned dopant materials may be
employed separately or in combination. Dopant material refers to
material which may alter physical and/or electrical properties of
the semiconductor layers 130, 270. Doped first cadmium telluride
layer 142 and intrinsic second cadmium telluride layer 144 each may
have a thickness of more than 1 nm, more than 10 nm, more than 20
nm, more than 1 .mu.m, more than 5 .mu.m, or less than 10
.mu.m.
[0027] The first dopant may be incorporated into the doped first
cadmium telluride layer 142 before, during or after deposition
using any suitable doping technique. For example, the first dopant
can be supplied from an incoming first dopant powder to be combined
with a material to be deposited such as cadmium telluride, a
carrier gas, or a directly doped powder such as a cadmium
telluride-silicon powder. Alternatively, the first dopant can be
supplied by diffusion from another layer of device 20. For example,
a dopant material such as rubidium within one absorber layer can
diffuse into another absorber layer. The first dopant concentration
in the doped first cadmium telluride layer 142 can be about
10.sup.-7% to about 10% by weight, about 10.sup.-5% to about
10.sup.-3% by weight, about 10.sup.-3% to about 0.1% by weight, or
about 0.1% to about 1% by weight. Depending on the rate of
incorporation of the first dopant into doped first cadmium
telluride layer 142, any suitable quantity of first dopant may be
introduced into a deposition environment to achieve such
concentration ranges, for example, more than 100 ppm, more than 250
ppm, more than 400 ppm, or less than 500 ppm.
[0028] After formation of cadmium telluride multilayer 270, one or
more heat treatment steps may be performed before a back contact,
such as back contact 150 in FIG. 1, is applied. Heat treatment
entails heat treating semiconductor-coated substrate with a
chlorine-containing compound, for example cadmium chloride, at
between about 380.degree. C. and about 800.degree. C., between
about 450.degree. C. and about 800.degree. C., or between about
380.degree. C. and about 450.degree. C., for about 20 minutes, for
example. Cadmium chloride can be applied by various techniques,
such as by solution spray, vapors, or atomized mist. Cadmium
chloride diffuses preferentially through grain boundary areas of
the intrinsic second and doped first cadmium telluride layers 144,
142, or interfaces where crystal grains or crystallites of
different orientations meet. Grain boundary areas typically contain
defects or other impurities, or atoms that have been disrupted from
their original lattice sites, which can reduce conductivity. This
process is known as healing or curing the grain boundary defects or
imperfections within layers 144, 142. During heat treatment,
recrystallization can occur, thereby enlarging cadmium telluride
grains and making a more uniform doping distribution within the
multilayer 270 possible. After healing layers 144, 142 through heat
treatment, photon-generated carriers, for example electrons and
holes, are more mobile and thus more easily collected.
[0029] FIG. 2B shows the device 20 after processing of the cadmium
telluride multilayer 270 is completed. A back contact 150 and a
back support 160, for example glass, are applied in sequence over
the cadmium telluride multilayer 270. The back contact 150 may
include one or more highly conductive materials. For example, the
back contact 150 may include molybdenum, aluminum, copper, silver,
gold, or any combination thereof.
[0030] FIG. 3A illustrates a second embodiment of the invention. In
FIG. 3A, a photovoltaic device 30 having a cadmium telluride
multilayer 370, which is similar to the multilayer 270 of FIG. 2A,
is depicted. However, in the cadmium telluride multilayer 370 of
the present embodiment, the intrinsic second cadmium telluride
layer 144 is formed between the window layer 130 and the doped
first cadmium telluride layer 142.
[0031] The photovoltaic devices 20, 30 in FIGS. 2A and 3A can
exhibit improved photo-conversion efficiency, for several reasons.
First, during heat treatment, the first dopant forms intermediate
compounds with low melting points, for example, a temperature below
a heat treatment temperature of about 450.degree. C., within the
window layer 130, within the absorber multilayer 270, and at the
interface between the window layer 130 and the absorber multilayer
270. The intermediate compounds melt during heat treatment. Such
compounds enable control over window layer 130 thickness in-situ
because the compounds cause the window layer 130 to flux or thin
but still allow window layer 130 to remain continuous and conform
to the front contact 120. This control is exercised through the
positioning of the doped first cadmium telluride layer 142 relative
to the window layer 130 and through the first dopant concentration
in the absorber multilayer 270. Such control reduces window layer
130 thickness thus mitigating the absorption of photons
therein.
[0032] Second, the FIG. 3A embodiment offers even greater control
over window layer 130 thickness in-situ because the intrinsic
second cadmium telluride layer 144 serves as a diffusion barrier
between the window layer 130 and the doped first cadmium telluride
layer 142. Therefore, the first dopant, rubidium or silicon for
example, must diffuse through the intrinsic second cadmium
telluride layer 144 to reach and react with the cadmium sulfide
window layer 130 to form the aforementioned intermediate compounds.
As a result, the window layer 130 is slower to flux. This delay can
provide for a wider temperature process window and increased
processing flexibility. For example, heat treatment can occur at
higher temperatures before the intermediate compounds form and
cause the window layer 130 to flux. Thus, window layer 130 thinning
still occurs which provides for mitigation of photon absorption
therein but it occurs in a delayed manner which allows for a more
flexible temperature window during processing.
[0033] Third, continuing reference to the FIG. 3A embodiment, in
addition to slowing first dopant diffusion into window layer 130
and for similar reasons, intrinsic second cadmium telluride layer
144 can also prevent excessive initial diffusion of first dopant
outside of doped first cadmium telluride layer 142 thus providing
for at least temporary first dopant concentration control within
the doped first cadmium telluride layer 142. A high dopant
concentration may increase the carrier concentration, e.g.
electron, hole, across the p-n junction at or near the interface of
the multilayer 370 and the window layer 130, which may lead to
increased photo-conversion efficiency.
[0034] It has been found that, after heat treatment, cadmium
telluride multilayers 270, 370 have had better grain structure and
surface roughness. For example, cadmium telluride multilayers 270,
370, each having the doped first cadmium telluride layer 142 with
first dopant silicon, demonstrated a surface roughness with a lower
standard deviation compared to conventional p-type absorber layer
140 (FIG. 1).
[0035] FIG. 3B shows the device 30 after processing of the cadmium
telluride multilayer 370 is completed. Back contact 150 and back
support 160, applied in sequence over multilayer 370, are identical
to such layers in the FIG. 2B embodiment.
[0036] FIG. 4A illustrates a third embodiment of a photovoltaic
device 40 having a cadmium telluride multilayer 470. After
formation of the intrinsic second cadmium telluride layer 144 over
doped first cadmium telluride layer 142, as described with respect
to FIG. 2A, at least one doped third cadmium telluride layer 146 is
formed over the intrinsic second cadmium telluride layer 144.
Cadmium telluride multilayer 470 can be formed through vapor
transport deposition, as shown in FIG. 7 and discussed below, for
example. The at least one doped third cadmium telluride layer 146
can have any suitable thickness, for example, more than 1 nm, more
than 10 nm, more than 20 nm, more than 1 .mu.m, more than 5 .mu.m,
or less than 10 .mu.m. The at least one doped third cadmium
telluride layer 146 can include a second dopant, for example a
Group I-B, V-A or VI-A dopant material such as copper, silver,
gold, nitrogen, phosphorus, arsenic, antimony, bismuth, oxygen
and/or chlorine-containing compounds of the above dopant materials.
As discussed in more detail below, the second dopant can be
different than the first dopant because the second dopant, for
example copper, minimizes the contact resistance (i.e., the
resistance of a material attributable to electrical leads and
connections) between cadmium telluride multilayer 470 and back
contact 150 and mitigates electron recombination at or near back
contact 150. The second dopant may also be the same as the first
dopant employed in the doped first cadmium telluride layer 142 or
may include the first dopant. The second dopant can be incorporated
into the at least one doped third cadmium telluride layer 146 using
any suitable doping technique such as those described with respect
to the first dopant (FIG. 2A). The concentration of the second
dopant within the at least one doped third cadmium telluride layer
146 can be 10.sup.-7% to about 10% by weight, about 10.sup.-5 to
about 10.sup.-3% by weight, about 10.sup.-3% to about 0.1% by
weight, or about 0.1% to about 1% by weight.
[0037] FIG. 4B shows the device 40 after processing of the cadmium
telluride multilayer 470 is completed. Back contact 150 and back
support 160, applied in sequence over cadmium telluride multilayer
470, are identical to such layers in the FIG. 2B embodiment.
[0038] FIG. 5A illustrates a fourth embodiment of a photovoltaic
device 50 in which the sequence of the doped first cadmium
telluride layer 142 and the intrinsic second cadmium telluride
layer 144 in FIG. 4A, can be reversed to form cadmium telluride
multilayer 570. After formation of the doped first cadmium
telluride layer 142, as described with respect to FIG. 3A, at least
one doped third cadmium telluride layer 146 is formed over the
doped first cadmium telluride layer 142. The advantages of the
second (FIGS. 3A and 3B) and third (FIGS. 4A and 4B) embodiments
discussed above, apply to the fourth embodiment.
[0039] FIG. 5B shows the device 50 after processing of the cadmium
telluride multilayer 570 is completed. Back contact 150 and back
support 160, applied in sequence over cadmium telluride multilayer
570, are identical to such layers in the FIG. 2B embodiment.
[0040] Photovoltaic devices 40, 50 with cadmium telluride
multilayers 470, 570 present several advantages. The incorporation
of the second dopant into the at least one doped third cadmium
telluride layer 146 widens the band gap adjacent to the back
contact 150 which, in turn, mitigates electron recombination at and
near the back contact 150. When photons are absorbed within the
multilayer 470, 570 electron-hole pairs generated in the multilayer
470, 570 are separated by the electric field at the p-n junction
formed at or near the interface of the multilayer 470, 570 and the
window layer 130. This creates electron flow toward such interface.
However, some electrons still may diffuse toward the back contact
150 where they can recombine with holes. The at least one doped
third cadmium telluride layer 146 treated with the second dopant
bends, i.e. widens, the band gap near the back contact 150 to
effectively repel electrons diffusing toward back contact 150 thus
protecting against electron recombination and increasing
photo-conversion efficiency. Additionally, the second dopant within
the at least one doped third cadmium telluride layer 146 results in
decreased contact resistance between the multilayer 470, 570 and
the back contact 150 as compared to conventional absorber layer 140
(FIG. 1).
[0041] FIG. 6A illustrates a fifth embodiment of a photovoltaic
device 60 having a cadmium telluride multilayer 670 which is
similar to the cadmium telluride multilayer 570 in FIG. 5A except
that at least one intrinsic third cadmium telluride layer 148 is
substantially free of dopant material, similar to the intrinsic
cadmium telluride layer 144. Forming the doped first telluride
layer 142 between the two intrinsic cadmium telluride layers, i.e.,
144 and 148, has advantages. First, as discussed above with respect
to FIG. 3A, the sequence of layers 144, 142 provides for delayed or
controlled fluxing of window layer 130 which mitigates photon
absorption within window layer 130 and also provides for a wider
process window, or renders window layer 130 less sensitive to
fluxing at high processing temperatures. Also, intrinsic layers
144, 148 serve as dual diffusion barriers such that layers 144, 148
contain a desired amount of the first dopant within the doped first
cadmium telluride layer 142 and prevent inter-diffusion up-and-down
to back contact 150 and front contact 120 thus providing for dopant
concentration control within multilayer 670. It has been found that
first dopant, for example rubidium or silicon, within concentration
ranges described above with respect to FIG. 2A, which can be better
achieved and maintained with the assistance of the barrier function
of layers 144, 148, can increase the free charge carrier
concentration in the window layer 130 and multilayer 670 which
increases the flow of electric current and improves the overall
electrical performance of the device 60.
[0042] FIG. 6B shows the device 60 after processing of the cadmium
telluride multilayer 670 is completed. Back contact 150 and back
support 160, applied in sequence over cadmium telluride multilayer
670, are identical to such layers in the FIG. 2B embodiment.
[0043] In general, fabrication of window layer 130 and respective
cadmium telluride multilayers 270, 370, 470, 570, 670 can be formed
using deposition system 70, as shown in FIG. 7. FIG. 7 illustrates
deposition system 70 for processing devices 20, 30, 40, 50, 60
which includes deposition stations 302, 312, 322, 332, 342, each of
which may include its own chamber. Alternatively, a single chamber
may house depositions stations 302, 312, 322, 332, 342 in
delineated areas, in which different materials may be deposited
under varying conditions. Each layer of devices 20, 30, 40, 50, 60
may be formed sequentially in respectively designated deposition
stations 302, 312, 322, 332, 342 in different stations or in the
same station according to the sequence described in the disclosed
embodiments.
[0044] Deposition stations 302, 312, 322, 332, 342 can be heated to
reach a processing temperature in the range of about 450.degree. C.
to about 800.degree. C. and can respectively include a deposition
distributor connected to a deposition vapor supply. Deposition
system 70 can include a conveyor 34, for example a roll conveyor
for conveying substrate 100 through deposition stations 302, 312,
322, 332, 342. The conveyor transports the substrate 100, e.g. a
soda-lime glass plate, along a transport path and into a series of
deposition stations 302, 312, 322, 332, 342 for sequentially
depositing layers of material on an exposed surface 32 of substrate
100. Each station 302, 312, 322, 332, 342 may have its own vapor
distributor and supply. The distributor can be in the form of one
or more vapor nozzles 36 with varying nozzle geometries to achieve
uniform distribution of the vapor supply.
[0045] By way of example, referring to FIGS. 4A and 7, window layer
130, doped first cadmium telluride layer 142, intrinsic second
cadmium telluride layer 144 and at least one doped third cadmium
telluride layer 146 can be respectively formed sequentially in
deposition stations 302, 312, 322, 332.
[0046] It should also be appreciated that substrate 100 depicted in
FIGS. 2A, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B and 7 may comprise one
or more layers, and may comprise any suitable substrate and base
materials. Thus, the deposition system 70 discussed and depicted
herein may be part of a larger system for fabricating a
photovoltaic device. Prior to or after encountering deposition
system 70, the substrate 100 may undergo various other deposition
and/or processing steps to form the various layers shown in FIGS.
2A, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B and 7, for example.
[0047] Although vapor transport deposition may be employed to form
window layer 140 and cadmium telluride multilayers 270, 370, 470,
570, 670, this is not limiting. Other suitable deposition
techniques may be used, for example atmospheric pressure chemical
vapor deposition, sputtering, atomic layer epitaxy, laser ablation,
physical vapor deposition, close-spaced sublimation,
electrodeposition, screen printing, spray, or metal organic
chemical vapor deposition.
[0048] The embodiments described above are offered by way of
illustration and example. It should be understood that the examples
provided above may be altered in certain respects and still remain
within the scope of the claims. It should be appreciated that,
while the invention has been described with reference to the above
example embodiments, other embodiments are within the scope of the
claims. It should also be understood that the appended drawings are
not necessarily to scale, presenting a somewhat simplified
representation of various features and basic principles of the
invention.
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